1. Technical Field
The present disclosure relates to a process of forming a nozzle opening for microfluidic and micromechanical chambers and, more particularly, to forming a nozzle with minimal amounts of gold.
2. Description of the Related Art
In applications using microfluidic structures or micro-electro mechanical structures (MEMS), fluid is often held in a chamber where it is heated. In addition, some fluids are processed at temperatures that need to be accurately regulated. The most common application is inkjet printer heads. Current inkjet technology relies on placing a small amount of ink within an ink chamber, rapidly heating the ink and ejecting it to provide an ink drop at a selected location on an adjacent surface, such as a sheet of paper. Other applications include analyzing fluids with organic components, such as enzymes and proteins, processing biological examinations, and amplifying DNA.
A DNA amplification process (PCR, i.e., Polymerase Chain Reaction process) is one process in which accurate temperature control, including repeated specific thermal cycles, needs to be carried out, while avoiding thermal gradients in the fluid. These organic applications require lower temperatures to process the fluid as compared to the high temperatures for inkjet printers. The different temperatures ranges are achieved by various combinations of microchip heaters.
Generally, generating local heat in a microchip includes heater elements positioned along one side of a chamber to be heated. The fluid is ejected from the chamber toward a target, which requires raising the temperature of the heater high enough to eject the ink and maintain the ink in a heated state as it exits the microchip. The chamber must then cool rapidly so that new fluid can be inserted into the chamber at liquid temperatures.
The current process of forming the ink chamber and nozzle includes forming a sacrificial oxide in a semiconductor wafer, the sacrificial oxide being approximately one micron thick and 200 microns wide. After formation of heater components, a large metal layer, such as gold is deposited and forms walls of the nozzle. The thick metal layer acts as a heat sink and prevents high temperatures from heater components from adversely affecting the durability of the inkjet cartridge or printer components. In some circumstances the heater temperatures may reach approximately 800 degrees Celsius.
The gold layer is approximately 17 microns thick, which corresponds to about 1.5 grams of gold per wafer and 40 grams per lot for 6 inch wafers. Accordingly, manufacturing large quantities of such devices requires large quantities of gold, significantly adding to the cost of manufacturing and the retail price of such devices. In addition, the process to form the large gold layer and define the nozzle is difficult and time consuming. Plus, the nozzle profile depends on the sensitivity of the photo-resist.
In addition to formation of the nozzle, front and back side protection layers are deposited to protect the substrate and device components while an inlet path and the final chamber are formed from the back side of the substrate. These processes complicate manufacturing and are difficult to control. The significant amount of gold, the application of the protection layers, and the sensitivity problem add to the cost of manufacturing and the ultimate retail price of such devices.